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Power Distribution and Protection Demonstrator

I design and build an educational tool to inspire young people to pursue a career in the electrical power sector through teaching core concepts of power distribution and protection in an engaging manner.

Parts list

Qty Product Part number
1 Eaton, SP 3 Position 45° Rotary Switch, 690V ac, 20A, Toggle Actuator 257-9637
1 Connector,audio/video,locking mains,chassis socket,3 way 246-8290
1 Neutrik Mains Connector Power Connector, 20A, Cable Mount, 250 V 246-8278
2 MK Electric MK white 1 Gang Plug Socket, 13A, Type G - British 500-0459
3 Schneider Electric, 110V dc Coil Non-Latching Relay DPDT, 12A Switching Current Plug In, 2 Pole 884-1424
3 Schneider Electric Relay Socket for use with Relais Series RSZ 8 Pin, DIN Rail, <250V 884-1297
1 Weidmüller Brown WSI Fused DIN Rail Terminal, 20 → 8 AWG, 250 V 042-5263
1 Weidmüller, W ATEX End Plate for Terminal Block 042-5291
1 WEW 35/2 335-4754
1 Neutrik, opticalCON QUAD Soft Plastic Cover for use with D-Size Chassis Connectors 771-0476
1 Neutrik, D Sealing Cap for use with D Series Connectors 121-7017
1 Arcol HS25 Series Aluminium Housed Axial Wire Wound Panel Mount Resistor, 15kΩ ±5% 25W 615-0454
1 Arcol HS100 Series Aluminium Housed Axial Wire Wound Panel Mount Resistor, 1.5kΩ ±5% 100W 252-2940
1 Arcol HS25 Series Aluminium Housed Axial Wire Wound Panel Mount Resistor, 3.9kΩ ±5% 25W 615-0432
2 RS PRO Switch Mode DIN Rail Power Supply with 100% Full Load Burn-In Test, 3-Years Warranty, Compact Size, Cooling by 136-8316
1 Rack Panel, 2U, Ventilated, Black, Steel 407-3887
1 Weidmüller Beige WDU Feed Through Terminal Block, 26 → 10 AWG, 4mm², ATEX, 800 V 042-5207
4 Tranilamp Mimic Indicator FYD/SEMMU/110DC/4
1 Multifunction Numerical Protection Relay Siemens 7SJ85
1 Multifunction Numerical Protection Relay Siemens 7SR51
1 Wera Interchangeable Hexagon; Slotted; Phillips; Pozidriv; Square; Three Square; Dual Bit; Tester Screwdriver Set 18 611-5855
1 Bahco Plier Type Rivet Tool 876-6531
1 RS PRO VDE/1000V Insulated 170 mm Straight Cutters 125-3077
1 RS PRO Natural Cable Tie Nylon, 300mm x 4.8 mm 489-2156
1 RS PRO VDE Insulated Steel Pliers Long Nose Pliers, 160 mm Overall Length 161-215
1 CK Cable Tie Gun, 2.4 → 4.8mm Capacity 188-5819
1 RS PRO Plier Crimping Tool for Terminal 053-3279

Thank you to RS Grass Roots for generously funding the purchase of tools used to build this project!


Society is increasingly reliant on electricity to meet its energy demands – the UK’s electrical infrastructure is constantly evolving and expanding to adapt to new needs and technologies as our reliance on fossil fuels decreases and the way that we generate electricity changes. For instance, the increasing popularity of fully electric propulsion rather than internal combustion engines in motor vehicles and the burgeoning transition from gas-fired heating to electrically powered heat exchanger systems are changing the dynamics of electricity demand.

The combination of a large-scale increase in the proportion of energy generated from renewable sources in recent years, an increase in electrical demand within the home, and changing load curves as more electricity is used for heating and electric vehicle charging signifies the need for change in the UK’s existing electrical power generation, transmission, and distribution infrastructure. These changes are conceived, designed, and implemented by power engineers.

The aim of this project is to design and construct a portable power distribution and protection system demonstrator for use in further, higher, and industrial education environments, with materials supplied by Siemens Digital Grid. More broadly, this project aims to provide a tool with which Siemens can inspire young people to pursue a career in the electrical power sector through teaching core concepts of power distribution and protection in an engaging manner during the critical early stages of the audience’s technical and professional development.

The demonstrator must give a realistic representation of the operation of real-world power distribution protection systems, feature interactive controls to facilitate a hands-on style of learning, be transportable by two persons in a typical hatchback car with the rear seats folded down, and be safe to operate.

Power Engineers: The Skills Shortage

A widely-documented skill shortage currently exists in the engineering industry. The 2021 Global Energy Talent Index (GETI) report found that in the power sector, “fifty-nine per cent of power professionals are worried that companies will not be able to fill essential roles”, and that with the COVID-19 pandemic, “the situation has become even more tricky” in a sector with a “retiring workforce” and “somewhat staid image”.

This is no new phenomenon: a BBC article from 2014 described a looming “retirement cliff” in the British engineering sector, with “the average age of an engineer in Britain” at that time being 54; the Energy Research Partnership published a report on their investigation into high-level skills shortages in the energy sector in 2007, the main findings of which include that “It is the shrinking pool of graduates that is at issue “, “The sector is seen to have a poor image among young people” and “A significant ‘outreach’ initiative could influence future career choices among young people”.

The 2021 GETI report also found that in the renewables sector, “fifty-seven per cent [of hiring managers] are worried about a potential skills gap”, an increase of 11% from the 46% stated in the 2019 GETI report. The summary of the 2021 GETI report notes that “The biggest threat of all remains a looming talent crisis”, and that “there are plenty of red flags around the entire energy industry’s ability to build a pipeline of qualified young people willing to join and remain in the industry”. Again, in 2014, the BBC found just 6% of UK students were studying “engineering or technology”.

Referring to the findings of these reports, it is imperative that organisations in the sector make meaningful efforts to reach out to young people to increase awareness of the diverse and worthwhile employment opportunities available: this approach directly combats the root of the talent shortage in the industry. This project aims to provide Siemens Digital Grid with a portable teaching tool for use in educational outreach programmes to inspire young people to become the next generation of power transmission, distribution, protection and automation engineers.

The Design



There are two main interactive sections to the demonstrator: feeder 1, which demonstrates overcurrent protection on a simple radial feeder using the IEC IDMT curve, and feeders 2 and 3, which demonstrate the breaker-and-a-half circuit breaker arrangement in a distribution context with two standard BS1363 13A mains outlets. A breaker-and-a-half arrangement allows two feeders to be run from either of two busbars and allows any one circuit breaker (CB) to be taken out of service or fail without interrupting the supply to either feeder.

A mimic panel showing a single-line diagram is fitted with indicators to give visual confirmation of the breaker statuses, and the measurements made by the protection relays may be observed in real time on their displays. The indicator lamps fitted to the mimic panel are special semaphore indicators, made by Tranilamp and sourced through RS components, which show circuit breaker position using two perpendicular LED bars: a red, vertical bar indicates the circuit breaker is closed, while a green, horizontal bar indicates the circuit breaker is open, as per the long-established industry-standard colour code for switchgear position status.

Feeder 1

The feeder supplies AC mains to a load bank representing loads of both less than the overcurrent pickup setting and greater than the overcurrent pickup setting. The load is selected using a 3-position rotary switch on the mimic panel. If the load current exceeds the current pickup, the CB will trip in accordance with the IEC standard inverse curve.

Feeders 2 and 3

The two BS1363 outlets allow the current characteristics of common household loads to be observed, along with the behavior of the breaker-and-a-half arrangement when an overcurrent fault occurs on one or both feeders.

Dimensions, Handling Ergonomics and Compatibility

The enclosure is designed to fit in the rear of a Volkswagen Polo and can be loaded by two persons. The enclosure is equipped with four handles for safe handling, one pair on either side. The handles are attached to the main body of the demonstrator, allowing the unit to be transported safely with the doors to the front and top rack sections removed, decreasing its weight. The demonstrator is designed to operate from a typical UK domestic mains supply via a single UK standard 13A-fused BS1363 plug.



The demonstrator must be safe for students to use, even in the absence of a supervisor. No live parts should be accessible, and all conductive parts are to be earthed where appropriate, including the earth terminals of the two BS1363 sockets. The base of the enclosure is fitted with anti-slip rubber feet to prevent sliding when the controls are operated - injury could be caused were it to fall. All screw fasteners were secured with thread-locking compound to mitigate loosening when the demonstrator is in transit.

The enclosure was designed to comply with the BS EN 60529 standard to ingress protection code IP2X, which means no solid foreign object over 12.5mm in diameter (e.g. a finger) may come into contact with live conductors without disassembly of the device. The demonstrator is fitted with a Neutrik PowerCON mains inlet, a 20A-rated locking connector widely used as a rugged alternative to the common IEC standard C14 mains inlet, which is rated at 10A (a lower rating than the 13A plug fuse utilised).

The two sockets are double-pole switched to completely isolate both loads when a trip occurs. This eliminates the hazardous condition of one conductor of each socket remaining connected to the supply in cases where the demonstrator is connected to an incorrectly wired socket. This possibility may be encountered when the demonstrator is ‘on the road’ – correct supply wiring should not be assumed when considering safe design of portable equipment. The two BS1363 outlets are fused at 3A, whilst the feeder 1 load bank is fused at 1A, which accommodates for inrush current. The fusing of the two outlets is supplementary protection - demonstration loads should already be appropriately fused within their plugs.

Enclosure Design


The unit is housed in a portable enclosure with removable doors for the front, rear and top of the enclosure. The front and top doors are removed to reveal a 19” rack of 12 rack units in size to the front face, and another 19” rack of 12 rack units in size to the top face. The rear door is left in place when the demonstrator is in operation but can be detached for maintenance access to reveal another 19” rack of 12 rack units in size, holding the load resistor plate and a length of BS EN 60715 ‘top-hat’ rail fitted with the 110VDC power supply unit, fuse holders, wiring terminal blocks and general purpose relays.

The rack on the top face of the enclosure is fitted with a mimic panel showing the single-line-diagram of the system with circuit breaker status shown via semaphore indicators, whilst the rack on the front face of the enclosure houses the protection relays and trip relays in a layout based on real-world protection cubicles I worked on during my year in industry. The trip relays and protection relays are mounted in standard 19” rack mounting frames, with the trip relays located beneath the protection relays associated with them. Blanking plates above each row of equipment allow equipment identification labels correlating to the single-line diagram shown on the mimic panel to be affixed above each relay.

Protection Relays and Trip Relays

Both protection relays utilised in the demonstrator are numerical protection relays from the current Siemens protection relay range – the Siemens Reyrolle 5 7SR51 relay used to protect the radial feeder is a flexible, if simple, multifunction protection relay designed for use in power distribution settings, whereas the Siemens SIPROTEC 5 7SJ85 relay used to protect the breaker-and-a-half arrangement is Siemens’ top-of-the-range overcurrent protection relay and is very much overkill in this application. These relays have been selected to show both the low-cost and high-cost options available in the Siemens protection range.

The trip relays selected are Siemens Reyrolle TR241 trip relays – these relays have two 110VDC coils, one puts the relay into the ‘tripped’ state, and another which electrically resets the relay to its un-tripped state, which is taken as the relay’s normal state. A resetting lever on the front of the trip relay allows the relay to be manually reset to its un-tripped state.

A red flag on the front of the trip relay falls into view when the relay has been tripped and remains visible until the resetting lever on the front of the trip relay is operated, even if the relay is electrically reset to the un-tripped state. This is a deliberate design decision by the manufacturer which persistently indicates a trip has taken place even if the circuit breaker is auto-reclosed to re-energise a circuit after a fault clears. This circumstance is common where the 79 Auto-Reclose function of the protection relay is utilised. The electric reset coils of the trip relays are connected to binary outputs on the protection relays but are not utilised in the configurations currently set – this is a deliberate decision to avoid confusing students unfamiliar with the function of the flag, which could easily be misinterpreted as an indication of the circuit breaker position.

A normally-closed contact on each trip relay feeds the coil of the relevant general purpose relay ‘circuit breaker’ – this means the general purpose relays are closed (and hence power is supplied to the load) only when the trip relay is in the un-tripped state. An opposing pair of normally-open and normally-closed contacts are used to illuminate the ‘open’ and ‘closed’ LED bars of the semaphore indicators to show whether the circuit breaker is open or closed.

Variable Load

The variable load for feeder 1 consists of three wirewound, aluminium-clad power resistors mounted to a large, earthed steel heatsink plate in the top of the rear of the case, below a louvred ventilation panel allowing air to circulate for cooling. A three-position rotary switch mounted on the mimic panel allows each of the three loads to be selected in turn. The resistances are selected such that the load power is as low as possible to minimise heat output whilst still allowing accurate measurement of the current drawn by the protection relay. It is not possible to touch the heatsink plate without first removing the rear cover of the demonstrator, mitigating the risk of users coming into contact with the warm plate.

The Build


First, the front rack of the enclosure was fitted with the label-holding blanking plates and the rack-mounting frames for the protection and trip relays. The three load resistors were fixed to a steel plate in the rear of the demonstrator, and a louvred vent panel was fitted above. Then, the trip relays were fitted to the lower frame, with the empty slots in the rack covered over with blanking plates. The SIPROTEC 5 7SR51 relay was fitted to the right-hand-side of the upper frame, and the plate was fabricated to fit the Reyrolle 7SR51 protection relay in the remaining space. The PowerCON mains inlet was riveted to a plate on the side of the enclosure. A length of top-hat DIN rail, fitted with the general purpose relays, fuseholders, terminal blocks and power supply unit, was fixed to the bottom of the rear rack. The semaphore indicators, BS1363 sockets and rotary switch were fitted to the mimic panel, and the mimic panel affixed to the top rack. Thread-locking compound was used on all machine screws. The wiring work was completed using a calibrated crimp tool. A multimeter was used to ensure earth bond continuity.


For feeder 1, the load currents measured by the protection relay were recorded for each switch position – from these values, the tripping times were calculated using the IEC standard inverse curve equation defined in IEC 60255. The trip times for each switch position were then measured experimentally with a stopwatch three times, and a mean time value to the nearest 0.25 seconds (to account for reaction time) was recorded in. The tripping times were precisely as expected.

For feeders 2 and 3, the current measurements shown on the relay display were the same as the multimeter-measured load current draws. Where a load current exceeded the 1A pickup for 1s or more, the trip relays associated with the load tripped, with the breaker positions shown on the mimic panel changing to ‘open’ accordingly. Where only one load exceeded the pickup, only the two breakers associated with the load were opened, leaving the other load powered. Where both loads exceeded the load current, all three breakers were opened, disconnecting both loads.

Where a trip occurred, the protection relay was reset by pressing the alarm reset button on the front panel, and the trip relays were reset using the manual reset levers. The mimic panel breaker positions returned to ‘closed’ and the supply to the load was restored. If the overcurrent fault condition were still present when the breakers were reclosed, the appropriate breakers would again trip to isolate the fault. The results were as expected – for loads drawing current less than the 1A pickup, no trip occurred, whereas for loads drawing current greater than the pickup, a trip occurred after a 1s time delay.


Operation of Demonstrator

The variable load of feeder 1 allows the difference in tripping time between currents slightly higher than the pickup and much higher than the pickup to be observed, which is ideal for teaching students the basic concept of an IDMT characteristic.

Examples of demonstration loads to be used with the breaker-and-a-half arrangement include a table lamp, a radio or a small motor – anything portable, safe and rated at less than 3A would suit. Loads with visual or auditory output would make demonstrations more compelling.

A suggested breaker-and-a-half demonstration involves a table lamp and a variable-speed fan with speed-dependent current draw. The current pickup and time delay settings for each feeder may be set independently on the relay front panel – the fan feeder pickup setting can be set between the fan’s current draw levels, such that the pickup current is exceeded if the speed is changed to a more powerful setting. If the current pickup setting for the lamp feeder is set higher than the draw of the lamp, it can be observed that tripping of the fan feeder does not necessarily result in isolation of the lamp supply. By shortening the lamp feeder time delay and setting the current pickup to just over the lamp’s steady-state current, it is possible to demonstrate the impact of over-sensitive protection settings in situations involving inrush current – the short delay and low current tolerance will likely result in the breaker tripping when the lamp is connected.

Scope for Additional Demonstrations

The reconfigurable nature of the demonstrator gives scope for a variety of alternative demonstrations, some examples of which are outlined in this section.

Implementation of Auto-Reclose on Feeder 1

For industrial education demonstrations where the audience is more familiar with the operation of protection systems, demonstrating the 79 auto-reclose function on feeder 1 could prove a novel and engaging demonstration.

As the electric reset coil of the trip relay is already wired to the protection relays’ binary outputs, configuring the 79 auto-reclose function and mapping the binary output connected to the electric reset coil would enable this demonstration.

Extension of Breaker-and-a-Half Logic

The 7SJ85 relay is configured in a somewhat naïve manner, where all circuit breakers are all assumed to be in functional and in service at all times. Additional logic would allow the behaviour of the system to be observed where a CB is taken out of service for maintenance, or where a CB fails, and an overcurrent fault occurs.

Implementation of IEC 61850

IEC 61850 communications between the two relays could be implemented easily for industrial education demonstrations – this is well beyond the scope of student demonstrations. Both relays are equipped with 61850-compatible ethernet modems.

Potential Improvements

A possible improvement to the demonstrator could be using a smaller, lighter-weight enclosure for greater portability. The demonstrator could also be split into two separate, smaller demonstrators. Substituting a steplessly variable load into feeder 1 would allow IDMT characteristics to be demonstrated more comprehensively.


The aim of producing a distribution and protection demonstrator for students has been met. The demonstrations possible with this unit are visually stimulating to increase engagement, and the reprogrammable design allows easy reconfiguration for additional demonstrations.

Thanks to the standard rack-mount case, the protection and trip relays may be substituted for newer models, increasing the life cycle of the unit. Siemens have expressed satisfaction with my design, implementation and standard of construction.

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